Is this route possible Xian - Hukou waterfall - Yan'an, with staying overnight in Yan'an?

One day is too tight for this route. You may have to stay overnight in Hukou.

Well, the direct bus from Xian to the Waterfall can be taken at the Fangzhicheng Bus Station and it takes around 5h. Buses are scheduled at 08:00, 13:20 and 14:20. And I remember the last bus from the waterfall to YA leaves at 14:00. You would fail to catch that. Then I suggest you spend a night there and catch the first bus back to YA in the next morning, at 07:30.

Thanks a lot. I saw that last bus from waterfall to Xian is at 17:40, so maybe we can change our direction, to go to YA first then waterfall and in the end Xian. Do you know maybe how to get to the waterfall from YA?

Hi Mariana. This is a good idea. The bullet train from Xian to YA only takes around two hours. At the opposite of YA Railway Station, it is the South Long-distance Bus Station. There you can take a direct bus to the waterfall in around 4h.
I have to remind you that in this way, you can cover the trip in a day, but you may have very limited for sightseeing.